Zika virus may be out of the headlines, but scientists are continuing to work on treatments and vaccines to address this serious threat to public health.

Now, an experimental vaccine against the virus has been shown to reduce the amount of virus in pregnant rhesus macaques and improve fetal outcomes. The study marks the first test of a Zika vaccine given before conception with exposure to the virus during pregnancy, said Koen Van Rompay, virologist at the California National Primate Research Center (CNPRC) at the University of California, Davis (UC Davis).

Zika virus infection of pregnant women is associated with a high risk of adverse fetal effects, including fetal death, microcephaly (small head) and other abnormalities, collectively termed congenital Zika syndrome. While no approved vaccine is currently available, the new study was designed to mimic a real-world scenario where women could be vaccinated months or years before becoming pregnant and be protected during pregnancy.

UC Davis researchers, alongside scientists from the National Institute of Allergy and Infectious Diseases (NIAI), injected female monkeys with candidate vaccine VRC5283. After vaccination, depending on their reproductive cycles, the female animals were housed with males and allowed to procreate. Thirteen vaccinated animals and 12 unvaccinated controls became pregnant. The investigators then exposed the pregnant animals to Zika virus at intervals representing first and second trimesters.

Two unvaccinated animals lost the fetus early in pregnancy due to Zika virus infection, but there was no early fetal loss in the vaccinated group. In addition, vaccinated females had less virus in their blood, and the virus persisted for a shorter duration after their exposure.

At the end of pregnancy, the researchers looked for Zika virus in tissues from the mothers and fetuses. It was found that 11 of 12 fetuses in the unvaccinated control group had detectable Zika virus RNA. However, no Zika virus RNA was detected in the 13 fetuses from the vaccinated group—suggesting that the vaccine prevented transmission of virus to the fetus. The results also indicate that VRC5283 may prevent mother-to-fetus transmission of Zika virus in humans, Van Rompay said.

The candidate vaccine is currently in trials, and results from the animal studies could help support the case for approving the vaccine.

Want to know more about the ongoing fight to eliminate Zika? Here are some additional ways NPRC scientists across the country are making progress against this disease.